This invention relates to a vacuum circuit interrupter and more particularly to the shield structure of a vacuum circuit interrupter.
FIG. 1 is a sectional view showing the structure of a conventional vacuum circuit interrupter disclosed in Japanese Utility Model Publication No. 53-43491, for example. In FIG. 1, the vacuum circuit interrupter comprises an electrically insulating tube 1 made of a glass or a ceramic material. A first flange 4 is attached to the upper end of the insulating tube 1 through a cylindrical sealing member 3, and a second flange 6 is attached to the lower end of the insulating tube 1 through a cylindrical sealing member 5. The first flange 4 has secured at its center a stationary electrode rod 8 having a stationary electrode 7 at its lower end, and the second flange 6 has secured at its center an axially expandable bellow 9, and the other end of the bellows 9 has mounted thereon a movable electrode rod 11 having at its tip a movable electrode 10 opposing the stationary electrode 7. The electrode rods 8 and 11 are axially aligned, and the insulating tube 1, the sealing members 3 and 5, the flanges 4 and 6, and the bellows 9 together constitute a vacuum vessel 12. A cylindrical main shield 13 of a circular cross-section is mounted at its central portion to the central portion of the insulating cylinder 1. Also, the upper and lower edges of the main shield 13 are inwardly rolled over. On the inner surface of the first flange 4 an outer shield 14 is provided, and on the upper surface of the second flange 6 an outer shield 15 is provided. Further, the outer shields 14 and 15 are of a cylindrical shape having an axial length slightly longer than that of the sealing members 3 and 5, and their end portions are bent inwardly to form concave surfaces at the portions facing the main shield 13. Also, between the end portions of the outer shields 14 and 15 and the opposite end portions of the main shield 13, a gap which is necessary for a withstand voltage and a gap which completely prevents the pollution of the insulating cylinder 1 resulting from the diffusion of the metallic vapor generated by the arc discharge are provided. Further, a bellows shield 16 surrounding the bellows 9 is mounted to the movable electrode rod 11.
With the conventional vacuum interrupter of the above-described structure, when the electrodes 7 and 10 are opened while an electric current flows through the electrode rods 8 and 11, an electric arc is generated across the electrodes 7 and 10. This arc melts the electrodes 7 and 10 and generates metal vapor to allow the vapor to diffuse into the vacuum sapce. In order to prevent pollution of the insulating vessel 1 by the metal vapor, the main shield 13 is provided thereby to trap most of the metal vapor. Further, the metal vapor which escapes from the upper and the lower ends of the main shield 13 is repelled back by the outer shields 14 and 15 and the flanges 4 and 6 to the inside of the main shield 13. This phenomenon occurs when the space between the electrodes 7 and 10 and the main shield 13 is large, and when the vacuum interrupter is very compact the arc generated across the electrodes 7 and 10 is driven to the outer periphery of the electrodes 7 and 10 by a magnetic field generated by the arc, often causing the main shield 13 to melt.
Since the conventional vacuum interrupter is constructed as described above, particles of the melted main shield 13 scatter in the axial direction of the main shield 13 and condense on the upper and the lower end portions of the main shield 13 and on the electrodes 7 and 10 when they reach the rounded portions. Therefore, the distances between the electrode 7 and the shield 13 as well as the electrode 10 and the shield 13 are shortened, decreasing the dielectric recovery characteristics during current interruption and the withstand voltage characteristics after currenet interruption.